Ad-hoc mobile networks are wireless networks capable of organizing themselves without any previously defined infrastructure. In an ad-hoc network, the radio communications medium is shared by the stations which are within mutual range of each other. The stations of the same network thus share between them the transmission medium so as to be able to emit or to receive packets. When two stations emit at the same time over a transmission medium, there exists a risk of collision.
A known random access protocol called CSMA/CA (acronym for Carrier Sense Multiple Access with Collision Avoidance) can be used for avoiding such collisions in the network. According to this access method, each station (otherwise known as a “node” hereinafter) is free to communicate when the transmission channel is free, in other words when no signal in progress is detected. Acknowledgements of receipt (ACK) are generally used for determining whether a transmission has failed, notably due to a collision. When such a situation occurs, the stations generally wait for a random time prior to re-starting the transmission, which reduces the probability of collision.
The CSMA/CA method is based on a mutual exclusion which may be expressed in the form of a conflict relationship between the links for access to a common resource. When the ad-hoc network uses protocols from the family of standards IEEE 802.11, it is assumed that a conflict relationship exists between the links connecting two stations whenever these links are sufficiently close, in the sense of the topology. More precisely, it is considered that a conflict relationship exists between two links (a,b) and (a′,b′) if the minimum distance in number of hops in the topology between the pairs of nodes {a,a′}, {a,b′}, {b,a′} and {b,b′} is less than or equal to a parameter N−1 (a, b, a′ and b′ denoting nodes). The parameter N represents a number of hops defined by the routing protocol and is such that each node can acquire the knowledge of the topology of the network up to at least N+1 hops.
The CSMA/CA random access method thus allows certain simultaneous transmissions that would risk interfering with each other to be avoided, and the retransmissions of messages to be limited. However, the CSMA/CA access method does not allow the common resource to be distributed equitably between the links in conflict. The result of this is shortages of resource over some links when other links in conflict with the latter are too heavily solicited.